1. Field of the Invention
The present invention relates to optical transmitter devices, and more particularly, to an optical transmitter device for controlling transmission of an optical signal.
2. Description of the Related Art
With the advance of multimedia, the development of optical communication networks is being furthered to realize high-speed, low-cost transmission of enormous amounts of information over long distances. One of the devices indispensable to such optical communication networks is optical modulator.
Optical modulators are roughly classified into the type which uses an external voltage to modulate optical intensity (corresponding to amplitude modulation of radio wave) and the type which uses an external voltage to modulate optical phase. In optical fiber communications, the intensity modulation type is most commonly used.
FIG. 19 shows the configuration of an optical modulator. The optical modulator 100 is an intensity modulator wherein a Mach-Zehnder (MZ) interferometer (optical interferometer so configured as to split input light into two beams and subsequently reunite the two beams), which is constituted by an optical waveguide 101, is formed on a crystal substrate of lithium niobate (LiNbO3: hereinafter “LN”) or the like having an electrooptic effect (change of the refractive index induced by application of an electric field).
The optical waveguide 101 diverges into two parallel waveguides 101a and 101b. As shown in FIG. 19, a signal electrode 102 is formed near the parallel waveguide 101a, and a ground electrode 103 is formed on both sides of the signal electrode 102 (FIG. 19 shows a Z-cut substrate with a single electrode structure).
Also, the signal electrode 102 is connected with a resistor R at the illustrated position and thus terminated. A predetermined voltage is applied to the signal electrode 102 so that the signal electrode 102 may act as a traveling-wave electrode which causes electrical and optical signals to travel in the same direction.
Due to the electric field (voltage) applied at this time to the optical waveguide 101, the refractive indexes of the parallel waveguides 101a and 101b change by +Δn and −Δn, respectively. As a result, the phase difference between the parallel waveguides 101a and 101b changes, and thus an intensity-modulated optical signal is output from the output waveguide (the intensity of the optical signal increases if the phase difference between the parallel waveguides 101a and 101b is 0° and decreases if the phase difference is π).
Thus, in the LN modulator having an optical waveguide formed on the LN crystal having an electrooptic effect, the refractive index of one optical path (optical path length) is changed to vary the interference state, thereby switching the optical signal ON and OFF. Also, the electrooptic effect takes place in a very short response time and permits high-speed modulation (e.g., at 10 Gb/s or higher).
FIG. 20 shows the configuration of a conventional optical transmitter device including the optical modulator 100. The optical transmitter device 110 comprises the optical modulator 100, a PD (Photo Diode) 111, an operation controller 112, and a bias T circuit 113.
The signal electrode 102 of the optical modulator 100 is input at one end thereof with an input data signal (“0”, “1”) via a capacitor C. The other end of the signal electrode 102 is connected to the bias T circuit 113 and a terminating resistor R, and applied via the bias T circuit 113 with a bias voltage generated by the operation controller 112.
The operation controller 112 has a low-frequency oscillator therein and superimposes a low-frequency signal generated by the oscillator on the bias voltage. Thus, the optical modulator 100 is driven by a signal which is derived by superimposing the low-frequency signal on the input data signal, to output an intensity-modulated optical signal.
The operating point for the optical modulation of the optical modulator 100 varies (drifts) depending on temperature or with time. To cope with such variation, the optical signal output from the optical modulator 100 is split by a coupler and converted to an electrical signal 111a by the PD 111, and based on the result of detection of the low-frequency signal contained in the electrical signal 111a, the operation controller 112 controls the bias voltage to an optimum value.
If the frequency component of pilot signal appears in the electrical signal 111a, then it means that the bias voltage is deviated from an optimum operating point, and if the frequency component of the pilot signal does not appear in the electrical signal 111a, it means that the bias voltage is optimized. Feedback control is carried out in this manner, thereby controlling the optical modulator 100 so as to always operate at a constant operating point.
As techniques applied to conventional optical transmitter devices including an optical modulator, there has been proposed a technique of cutting off the optical signal output from the optical modulator when a power alarm or a wavelength alarm is received (e.g., Unexamined Japanese Patent Publication No. H11-340919 (paragraph nos. [0036] to [0044], FIG. 5)).
In optical fiber communication systems, shutdown control for automatically stopping a high-level optical output (called “APSD (Auto Power Shut Down)”) is performed in order to protect the human body or prevent a fire in case an optical fiber connector comes off or an optical fiber becomes disconnected, or at the time of line switching of the system.
In the case of carrying out the shutdown in the aforementioned optical transmitter device 110, the operating point of the optical modulator 100 is changed in response to a shutdown instruction from the host side, to lower the optical output level.
According to the conventional shutdown control, however, a long time is required after the operation controller 112 receives a shutdown instruction until the optical output of the optical modulator 100 actually drops, with the result that the shutdown cannot be performed at high speed.
In the operation controller 112, the electrical signal is filtered to extract the low-frequency signal, and a phase comparator compares the phase of the extracted low-frequency signal with that of the low-frequency signal generated by the low-frequency oscillator. The phase comparator outputs the derived phase difference component as a pulse-like phase difference signal, and a loop filter smoothes the phase difference signal (turns the signal into direct current) and amplifies the resultant signal, thereby generating the bias voltage.
On receiving a shutdown instruction, the operation controller 112 controls the internal elements to output a bottom voltage necessary for lowering the optical output level of the optical modulator. However, the aforementioned loop control requires a certain period of time for the loop, and in addition, the loop filter has a certain time constant necessary to stabilize the loop control (the response is delayed for a time period corresponding to the set time constant). Consequently, the currently output voltage cannot be switched at high speed to the bottom voltage after the reception of a shutdown instruction, making it impossible to instantly complete the shutdown.
In recent years, optical fiber communication technologies enabling high-speed, large-capacity optical communication of the order of 10 Gb/s or even 40 Gb/s, for example, are developed, necessitating correspondingly high-speed line switching. Accordingly, there is a strong demand for techniques that enable the optical transmitter device itself to shut down at high speed.
According to the aforementioned conventional technique (Unexamined Japanese Patent Publication No. H11-340919), the optical output from the optical modulator is simply stopped on reception of an alarm, which is the condition for stopping the optical output, and no consideration is given to high-speed shutdown control.